Hi Paul,
> W8JI:
>
> > The ideal shape is a raised sine wave shape, not the "perfect"
> > shape they show.
>
> I suppose it depends on your definition of the perfect wave shape. If
> what you see is the same as my reference on p 15.6 of the 1998 ARRL
> Handbook, I would gladly trade my eye teeth for the waveform as
> represented in figure 15.8(a). This represents the classic grid-block
> keying waveform as used with simple single-pole R/C timing circuits.
> Here, the leading edge rises quickly and gradually decelerates until the
> final envelope is formed. The trailing edge falls quickly but then trails
> asymptotically to zero. It's this waveform that carries that plosive,
> bell-like sound that's so unmistakable through selective fading.
If the goal is to copy the signal as best as possible at the highest
speed possible while not disturbing others, then the ARRL's
waveform is certainly not ideal. It is the product of cheap casual
engineering or engineering practical in tube-type rigs with grid-block
keying, not what we have been very easily able to do for a few
dollars since the early 70's when op-amps came available.
Such a response can be likened to a 10Hz to 10,000Hz response
on a SSB transmitter. It might stir "warm and fuzzy" feelings in
some people, but it wastes spectrum and does not add to
readability.
> 10K control pot with a fixed resistor in its place. On the other hand,
> the keying on a stock Yaesu FT-1000D sounds much too soft (yes, yours too
> Tom). But the FT-1000MP is just the opposite.
Yet people who copy me near the noise floor always report no
change in readability when I go to a hard clicky waveform in blind
tests. So why should I click if it doesn't help signal readability?
That's the same reason I won't use a 3.5kHz SSB filter.
It makes me believe that
> little thought is given to the keyed wave-form in today's transceivers
> when they gladly accept any result even if it varies wildly from one
> generation of transceiver to the next...especially within the same
> manufacturer. The mind-set appears to be "why bother with CW keying
> improvements when that cost can be spent on adding more useless features."
I agree with that. The hardest possible waveform for a given
bandwidth comes with a raised-sine waveform.
> It's true that CW bandwidth is a function of the envelope rise and decay
> time, but more than one type of wave-shape can produce an equal level of
> occupied bandwidth. The "perfect" wave shape is purely subjective.
> Provided that both rise and decay time is greater than 2 mS or so,
> occupied bandwidth should be reasonable...reasonable in that the signal is
> easy to copy during QRM and selective fading, and reasonable in that the
> minimum amount of bandwidth is required for copy at the receiving end.
Not so. The slope of the waveform at any point controls bandwidth.
I can have a 2mS rise and fall that clicks like hell 5 kHz away, and
I can have one that is not bothersome at all 500Hz away.
Time is meaningless by itself, the important thing is the slope of
the envelope over time. The very last thing you want is ANY abrupt
edge or corner in the waveform at any point, because that is what
causes the bandwidth problem.
If the problem in the Ten Tec is only caused by ALC reducing
limiting rise time, it could be easily cured by adding an external
negative FIXED ALC voltage to reduce gain. The FT1000D does
that with a front panel drive control. We need a law requiring a drive
adjustment on rigs, so the closed ALC loop does not have to limit
gain over such a wide range.
> edition of QST. (See pp. 11-15, 166). George developed a unique keying
> circuit in that two controls are brought out from the transmitter to
> *separately* adjust the wave-form rise and decay time. Keying could then
> be adjusted to conform to band conditions. Limits placed on both controls
Unless George filtered the bandwidth of the applied keying
waveform in a low-pass filter, he did not secure minimum bandwidth
for a given rise and fall. Length of rise and fall is all but
meaningless unless it includes a sharp cut-off filter and is applied
through a linear AM modulation scheme.
There is also a second problem that is almost always overlooked.
Some rigs switch into transmit while the synthesizer is still moving
to a new frequency, while others switch before the thing even
moves! One popular high-end DSP rig does that, you can tell them
in pile ups because they are the rigs that thump on top of the DX
station when they are working split frequency.
73, Tom W8JI
W8JI@contesting.com
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